The line under the sideways
∪
means that
A
may also be equal to
B
(that is, they may be identical sets). If we want to say that
A
is a proper subset of
B
(that means: it's a subset, but there is at least one element in
B
that is not in
A
) then we can remove the line:

A
⊂
B

To write that a set is not a subset of another set, just put a slash through the sideways
∪
: